The neglected tropical disease Schistosomiasis is the most socioeconomically devastating helminth infection, and the second most burdensome parasitic infection behind malaria, infecting over 200 million people worldwide (Colley et al., 2014). Clinical outcomes span gastrointestinal and liver pathologies, genitourinary disease, anemia, undernutrition, growth retardation and a heightened risk for comorbidities. Infection increases the risk of HIV transmission, making effective drug therapy for schistosomiasis a healthcare priority. Overall, the schistosomiasis disease burden encumbers third world economies with an annual loss of up to 70 million disability-adjusted life years. Infected individuals are treated by the drug praziquantel (PZQ), the mainstay therapeutic for disease control.
PZQ was originally developed during the 1970s, and the continued effectiveness of this agent over four decades of usage for treating a variety of parasitic infections has proven critically impactful (Colley et al., 2014). Indeed this clinical efficacy has ironically proven to be a factor that has restrained efforts to develop alternative therapies, and at the most basic level, define how PZQ works. However, several features of PZQ remain less than ideal and require improvement. First, the lack of a mechanistic understanding of how PZQ works has proved a roadblock in the rational design of new drugs. There is a need to identify new druggable targets that exploit broader vulnerabilities within PZQ-sensitive pathways (Chan et al., 2013; Salvador-Recatala and Greenberg, 2012; Aragon et al., 2009). Second, the inability to improve on PZQ by chemical derivatization of the drug. All PZQ derivatives synthesized to date are less effective than the parent compound. The need is to identify novel structural pharmacophores that impair parasite viability. Third, the inability of PZQ to kill all parasitic life cycle stages. Juvenile worms are refractory to PZQ (Greenberg, 2013; Hines-Kay et al., 2012), possibly a contributory factor driving development of drug resistance (Greenberg, 2013; Wang et al., 2012). The need is to identify new targets expressed throughout all lifecycle stages that are ideally conserved in other PZQ-sensitive parasites. Fourth, sub-optimal cure rates in the field: PZQ requires multiple drug dosings to achieve maximal cure rates for schistosomiasis, a regimen which is not always executed in mass drug administration efforts (King et al., 2011; Olliaro et al., 2014). Therefore, there is clear opportunity to improve on the clinical penetrance of PZQ. These issues support efforts to identify new, druggable targets for development of next generation anthelmintics.
During regeneration of the planarian flatworm D. japonica—a widely used regenerative biology model (Newmark and Sanchez-Alvarado, 2002)—PZQ miscued polarity signaling to cause regeneration of bipolar (‘two-headed’) worms with dual, integrated organ systems (Nogi et al., 2009). This visually striking phenotype, coupled with the tractability of the planarian system to in vivo allowed the pathways engaged by PZQ in vivo to be defined (Nogi et al., 2009; Zhang et al., 2011; Chan et al., 2015; Chan et al., 2014). These studies culminated in a model where PZQ acts as an ergomimetic (Chan et al., 2015) with in vivo PZQ efficacy regulated by the opposing functionality of dopaminergic and serotonergic neurons (Nogi et al., 2009; Zhang et al., 2011; Chan et al., 2015; Chan et al., 2014), known regulators of muscular activity, the tissue where planarian polarity determinants reside (Witchley et al., 2013). The serotonergic and dopaminergic G protein coupled receptors (GPCRs) engaged by activity of these bioaminergic neurons therefore represent potential downstream PZQ effectors.
This is an important realization as flatworm G protein coupled receptors (GPCRs) are logical candidates for antischistosomal drug development efforts. Over one quarter of current therapeutics target rhodopsin-like GPCRs (Overington et al., 2006). However, barriers have been a lack of understanding of the physiology of specific GPCRs from within the broad GPCR portfolio (about 75-120 in S. mansoni (Campos et al., 2014; Zamanian et al., 2011; Berriman et al., 2009)) expressed by these organisms, as well as struggles optimizing functional expression of individual flatworm GPCRs in heterologous assay systems. However, several groups have now begun to define a role for specific GPCRs within the chemotherapeutically vulnerable excitable cell niche (Chan et al., 2015; Patocka et al., 2014; El-Shehabi et al., 2012; MacDonald et al., 2015), highlighting the key challenge of optimizing robust platforms for pharmacologically profiling these GPCRs in a miniaturized format compatible with high throughput screening (HTS). Prior studies have simply relied on interrogation of expressed GPCRs against handfuls of ligands selected around inferred agonist specificity.